The methods disclosed in NPL 1 and NPL 2, which have been standardized by ITU-T, are known as coding schemes enabling efficient coding of sound-related data such as speech data in the Super-Wide-Band (SWB, usually a band of 0.05-14 kHz). In these methods, sounds in a band of 7 kHz or lower (hereinafter referred to as a “low band”) are encoded by a core coding section and sounds in a band of 7 kHz or higher (hereinafter referred to as an “extension band”) are encoded by an extension coding section.
CELP (Code Excited Linear Prediction) is used in coding processing by the core coding section. The extension coding section decodes a low-band signal encoded by the core coding section, transforms it into the frequency domain by using MDCT (Modified Discrete Cosine Transform), and makes use of the obtained spectra (or transform coefficients; hereinafter referred to as “transform coefficients”) in encoding in the extension band.
The extension coding section uses the “envelope” of spectral power to normalize the core encoded low-band transform coefficients generated by the core coding section. In particular, the extension coding section calculates energy in each subband, smoothens out the subband energy to make a variation of the energy smooth in the direction of the frequency domain, and normalizes the transform coefficients in each subband with the smoothened energy. The normalized transform coefficients obtained in this manner are hereinafter referred to as “normalized low-band transform coefficients.”
The extension coding section searches for a subband having a large value of correlation between the normalized low-band transform coefficients and transform coefficients from an input signal in the extension band (hereinafter referred to as “extension-band transform coefficients”) and encodes information indicating the subband as lag information. The extension coding section copies the normalized low-band transform coefficients in the subband having a large value of correlation to the extension band and utilizes the copied normalized low-band transform coefficients as a spectral fine structure of the extension band. Thereafter, the extension coding section calculates a gain to adjust energy of the extension-band transform coefficients and encodes the gain. The coding apparatuses according to the related art perform the above-described processing to generate transform coefficients in the extension band using transform coefficients in the low band.
The value of correlation between the normalized low-band transform coefficients and the extension-band transform coefficients is calculated in the following manner in NPL 1 and NPL 2.
First, extension band is divided into a plurality of subbands (hereinafter referred to as “extension-band subbands”). Next, for each extension-band subband, a value of correlation between the normalized low-band transform coefficients and the transform coefficients in the extension-band subband is calculated. Then, a position of the normalized low-band transform coefficients where the value of correlation with the extension-band subband becomes largest is searched. However, calculating the value of correlation in this manner has a problem in that the method involves a large amount of calculation because the normalized low-band transform coefficients and all the transform coefficients in the extension-band subband are used for the calculation.
As a solution to this problem, PTL 1 discloses a technique in which the value of correlation is calculated by using only large transform coefficients in terms of amplitude among the extension-band transform coefficients. Accordingly, the amount of calculation for calculating the value of correlation can be reduced by limiting the number of transform coefficients used in the calculation of the value of correlation.